In external radiation therapy, converging beams of radiation are used to irradiate cancerous tissue within a patient.
Tomotherapy is a form of external radiation therapy in which the radiation source is placed on a gantry rotating in a single plane about an axis through the patient. The patient may be translated across the plane during the rotation to impart a relative helical motion between the patient and a point on the gantry. During this rotation, the radiation beam is modulated by a multi-leaf collimator (MLC) or other modulating device which divides the radiation beam into independently controllable rays. By controlling the intensity of each ray as a function of gantry angle, radiation dose may be precisely placed in arbitrary cross-sectional regions within the body. Methods of constructing and of operating such tomotherapy equipment are described in U.S. Pat. No. 5,317,616 issued May 31, 1994 entitled Method and Apparatus for Radiation Therapy, and U.S. Pat. No. 5,548,627 issued Aug. 20, 1996 entitled Radiation Therapy System With Constrained Rotational Freedom as assigned to the assignees of the present application and hereby incorporated by reference.
Tomotherapy""s ability to precisely place a radiation dose makes it important to be able to accurately image the treatment region and precisely locate the patient during treatment. U.S. Pat. No. 5,724,400 issued Mar. 3, 1998 entitled Radiation Therapy System With Constrained Rotational Freedom also assigned to the assignees of the present invention and hereby incorporated by reference describes a combination computed tomography (CT) machine and radiation tomotherapy machine that can provide both imaging and accurate registration of the patient.
In CT, a planar beam of kilovolt energy x-rays are projected through the patient at a variety of gantry angles much like that done with tomotherapy but without modulation of the beam rays by an MLC. Bodily structures attenuate the rays and this attenuation is detected by a detector. A cross-sectional image may be reconstructed from xe2x80x9cprojectionsxe2x80x9d of attenuation data of rays at each gantry angle over a range of gantry angles that define a xe2x80x9ctomographic projection setxe2x80x9d. The tomographic projections set typically includes rays spanning the entire width of the patient over at least 180xc2x0 of gantry angles. Reconstructing an image from less than a tomographic projection set can cause severe image artifacts obscuring essential body structure.
While normally CT uses kilovoltage x-rays having much lower energy than those used in radiation therapy, it is known that images can be constructed using the same megavoltage x-rays used in radiation therapy. In this way the need for a separate kilovoltage x-ray source is avoided. Megavoltage images have the further advantage of better representing the actual absorption of the body structures of radiation at the megavoltage lever thus making those images superior for treatment planning and dose verification. U.S. Pat. No. 5,673,300 issued Sep. 30, 1997 entitled Method of Registering A Radiation Treatment Plan To A Patient teaches methods of using a detector with megavoltage x-rays to verify patient location and for dose evaluation. These patents are also incorporated by reference.
Nevertheless, the use of megavoltage x-rays for the generation of a CT image (MVCT) has some disadvantages. First, the requirement that a complete tomographic projection set be obtained normally requires a pre-scanning of the patient before radiation therapy. This pre-scanning process increases the total time of the treatment and entails the possibility of patient movement between the scan and therapy. Pre-scanning typically forgoes the ability to use the MVCT images for real-time correction or verifications. Finally, high contrast MVCT over the required tomographic projection set significantly increases the dose to the patient.
The present invention eliminates or reduces the amount of additional time required to take an MVCT scan and/or minimizes the additional dose required by MVCT by collecting at least a portion of the CT data during the radiation therapy. The problem of image artifacts resulting from the inevitably incomplete projections provided by the modulated radiation therapy radiation, is overcome by supplementing this xe2x80x9chigh fluxxe2x80x9d radiation therapy data with low flux data obtained separately either in a pre-scan or during the radiation therapy. A tomographic projections set is formed of mixed low and high flux data.
Using low flux data minimizes the extra radiation to the patient required for MVCT whereas the high noise inherent in using low flux data is substantially corrected by the high flux data obtained from the therapy. Alternatively in certain situations, the low flux data alone may be used.
Specifically then, the present invention provides a combination radiation therapy and tomographic imaging machine having a megavoltage radiation source directing a radiation beam formed of rays directed generally along an axis. A gantry holds the radiation source to rotate the angle of the axis about a treatment volume and a radiation detector is positioned on the gantry across the treatment volume opposite the radiation source to provide data of a projection. A modulator is positioned between the radiation source and the treatment volume to modulate the flux of the rays.
An electronic computer communicating with the modulator controls the modulator to direct selected high flux rays at selected axis angles at the patient within the treatment volume according to a radiation treatment plan. The selected rays and angles comprise less than the tomographic projection set and thus produce a high flux, incomplete tomographic projection set. Low flux rays are also directed at the patient within the treatment volume to acquire low flux data. The low flux data is combined with the high flux incomplete tomographic projection set to produce an augmented but complete tomographic projection set which may be reconstructed into an image.
Thus, it is one object of the invention to employ radiation used in radiation therapy for CT imaging even though it is seriously incomplete for tomographic purposes. The low flux data augments the incomplete high flux radiation therapy data to provide a tomographic image without overly increasing the total dose to the patient.
The low flux data that is collected may be a complete tomographic projection set or may be an incomplete tomographic projection set.
Thus it is another object of the invention to provide great flexibility in acquisition of the low flux data.
In one embodiment, the modulator is a multi-leaf collimator having multiple leaves that may be opened or closed to pass or occlude rays and the low flux data are obtained from leakage through the leaves when they are closed.
Thus it is another object of the invention to provide a simple method of producing low flux data in a radiation therapy machine.
The leaves may be open for a proportion of time during increments of axis angle according to the radiation treatment plan to produce the high flux incomplete tomographic projection set and the electronic computer may execute the stored program to position the proportion of time in the open state after the beginning of each angular increment so that the low flux data may be collected at the beginning of each angular increment.
Thus it is another object of the invention to provide a ready means of obtaining a complete tomographic projections set of low flux data at regular intervals during the radiation therapy itself.
Alternatively, the low flux rays may be obtained by opening the leaves to pass rays not required by the radiation treatment plan.
Thus it is another object of the invention to provide a means of obtaining a complete tomographic projections set of low flux data.
The electronic computer may execute the stored program to center the proportion of time the leaves are in the open state within each angular increment and the low flux data may be collected at the center of each angular increment. Alternatively, or in addition, the computer may optimize the pattern of opening and closing the leaves according to the radiation treatment plan, presupposing an opening of leaves necessary to generate the low flux data.
Thus, it is another object of the invention to obtain tomographic projection set data at times where the greatest amount of flux associated with radiation therapy occurs so that a complete projection can be obtained with the least additional dose.
The electronic computer may further operate to control the modulator to direct selected high flux rays at selected axis angles at the treatment volume without the patient according to the radiation treatment plan to acquire a normalizing high flux incomplete tomographic projection set (xe2x80x9chigh flux air-scanxe2x80x9d) and to direct low flux rays at the treatment volume without the patient to acquire normalizing low flux data from the radiation detector (xe2x80x9clow flux air scanxe2x80x9d). Prior to combining the high flux incomplete tomographic projection set and the low flux data to produce the augmented tomographic projection set, the high flux incomplete tomographic projection set may be normalized with the normalizing high flux incomplete tomographic projection and the low flux data may be normalized with the normalizing low flux data.
Thus it is another object of the invention to better accommodate data of widely varying flux rates into a single tomographic projection scan without introducing extreme artifacts. The air scans provide common units of attenuation that may be combined between high and low flux data. When the modulator includes leaves, as described above, which may move between states to occlude and pass given rays where at any given time, a leaf may be stationary or moving and the data acquired from the radiation detector for reconstruction may be obtained only at times when the leaves are stationary.
Thus it is another object of the invention to provide a method of obtaining repeatable air scans and conventional scans using an MLC as is necessary for the combination of high and low flux data. The inventors have determined that sufficient repeatability can obtained using mechanical MLC through this windowing process.
The foregoing and other objects and advantages of the invention will appear from the following description. In this description, reference is made to the accompanying drawings, which form a part hereof, and in which there is shown by way of illustration, a preferred embodiment of the invention. Such embodiments and their particular objects and advantages do not define the scope of the invention, however, and reference must be made therefore to the claims for interpreting the scope of the invention.